A comparison of ICRU point doses and volumetric doses of organs at risk (OARs) in brachytherapy for cervical cancer.

INTRODUCTION: In brachytherapy for cervix cancer, doses to organs at risk (OARs) are traditionally calculated using the ICRU-38 point doses to rectum and bladder. Three-dimensional image-guided brachytherapy allows assessment of OAR dose with dose volume histograms (DVHs). The purpose of this study was to analyse the correlation between DVHs and ICRU point doses. METHODS: Using the PLATO™ planning system, the bladder, rectum and sigmoid were retrospectively contoured on 62 CT datasets for 20 patients treated with definitive radiotherapy. The median external beam radiotherapy dose was 45 Gy. Brachytherapy was delivered using a CT-MRI compatible tandem and ovoids to a median dose of 24 Gy in three fractions. DVHs were calculated, and the minimum dose to 2 cc of tissue receiving the highest dose (D(2cc) ) was recorded and compared with the ICRU point doses (D(ICRU) ). RESULTS: The mean rectal D(ICRU) was 4.01 Gy compared with D(2cc) of 4.28 Gy. The mean bladder D(ICRU) was 6.74 Gy compared with D(2cc) of 8.65 Gy. The mean sigmoid D(2cc) was 4.58 Gy. The mean dose ratios (D(2cc) /D(ICRU) ) were 1.08 for rectum and 1.39 for bladder. D(ICRU) correlated with D(2cc) for rectum (r = 0.76, P = 0.001) and for bladder (r = 0.78, P = 0.01). CONCLUSION: OAR doses assessed by DVH criteria were higher than ICRU point doses. The significant correlation between D(2cc) and D(ICRU) has allowed us to set DVH dose constraints for CT-based brachytherapy and thus begin the transition from two-dimensional to three-dimensional image-guided brachytherapy planning.

A basic treatment equivalent for high-dose-rate gynaecological brachytherapy--a pilot study.

BACKGROUND & PURPOSE: The aims of this pilot study were to assess the factors that were predictive of high-dose-rate (HDR) gynaecological brachytherapy duration and to model them using previously described Basic Treatment Equivalent (BTE) methodology. MATERIALS AND METHODS: This was a prospective single arm pilot study that aimed to enrol 20 patients from two centres. Patient, tumour and treatment factors were recorded. The duration of each component of brachytherapy was recorded. Univariate and bivariate analyses were conducted to identify factors that predicted overall brachytherapy duration. The generalized estimating equations method was used to derive an equation that predicted the duration of brachytherapy. RESULTS: Data were collected for 20 patients who underwent 53 episodes of brachytherapy, either as part of definitive radiotherapy (tandem and ovoids) or post-operatively (vaginal cylinder). Factors that were predictive of overall duration were technique (tandem and ovoids vs vaginal cylinder, P = 0.0007), treatment intention (definitive vs post-operative, P = 0.0001), type of plan (individual vs standard, P = 0.0001), hospital (1 vs 2, P = 0.0001) and body mass index (P = 0.0001). CONCLUSIONS: This study demonstrates the feasibility of examining factors that influence the duration of gynaecological brachytherapy using BTE methodology. To develop a reliable model, a larger multicentre study is needed. Such a model will allow comparisons of efficiency and more accurate assessment of treatment capacity between centres.